The present invention relates to a multi-beam exposure apparatus scanning a plurality of light beams and usable for a plural drum type color printer apparatus, a plural drum type color copier, a multicolor printer, a multicolor copier, a monochromatic high speed laser printer, a monochromatic high speed digital copier and the like.
For example, in an image forming apparatus such as a color printer apparatus, a color copying apparatus which employs an image forming unit including a plurality of photosensitive drums, there is employed an exposure apparatus which supplies a plurality of light beams having the number the same as the number of plural image data corresponding to separated color components, that is, at least the number of image forming units.
This kind of exposure apparatus has a plurality of semiconductor laser elements emitting a predetermined number of light beams corresponding to the image data at each of the separated color components, a first lens group stopping down a cross sectional beam diameter of the light beam after each of the semiconductor laser elements is emitted to a predetermined size and shape, an optical deflecting apparatus operated in accordance that a recording medium holding the image formed by each of the light beams continuously reflects in a direction perpendicular to a transferred direction, a second lens group image forming the light beam deflected by the optical deflecting apparatus on a predetermined position of the recording medium, and the like. Here, in many cases, a direction in which the laser beam is deflected by the optical deflecting apparatus is indicated to be a main scanning direction, and a direction in which the recording medium is transferred, that is, a direction perpendicular to the main scanning direction is indicated to be a sub scanning direction.
The exposure apparatus mentioned above is classified into an embodiment which employs a plurality of exposure apparatuses corresponding to each of the image forming units according to the applied image forming apparatus and an embodiment which employs a multi-beam exposure apparatus capable of supplying a plurality of light beams by means of one exposure apparatus. In this case, in recent days, in order to increase an image forming speed and improve a resolution, there has been also proposed a high speed printer apparatus capable of forming an image having a high resolution and forming an image at a high speed by exposing image data having the same color in a parallel manner.
However, in the exposure apparatus mentioned above, when a rotational speed of the reflection surface of the optical deflecting apparatus is increased in order to increase the speed of the image forming apparatus and improve the resolution, it is necessary to employ an expensive bearing such as an air bearing which can stand against the high speed rotation on the reflection surface. On the other hand, as well as the rotational speed of the motor has an upper limit, the motor capable of rotating at a high speed is expensive and a drive circuit for rotating the motor is also expensive, so that there is a problem that an increase of the rotational speed of the reflection surface corresponding to an increase of the cost can not expected. In this case, when the rotational speed of the reflection surface is increased, as a result, there is generated problems that the wind sound is increased as well as the wind damage on the reflection surface is accelerated.
On the other hand, it is possible to restrict an increase of the rotational speed of the motor when increasing the number of the reflection surfaces, however, since an image frequency is increased, there is a problem that a noise component superimposed on the image signal (image data) at a high possibility is increased. Further, when the image frequency is increased, there is a problem that various kinds of limitations are generated in designing or mounting the control circuit.
Accordingly, there has been proposed a multi-beam exposure which can reduce each of the rotational speed of the reflection surface and the image frequency by allocating a plurality of light beams to each of the separated color components and deflecting (scanning) them at one time, however, even in the case of employing the multi-beam exposure, there are various kinds of problems as mentioned below.
That is, in the multi-beam exposure, there is employed a method of using a plurality of light sources for each of the separated color components and combining the light beams emitted from each of the light sources at a color component unit so as to deflect (scan) as one light beam, and a semiconductor laser element is employed for the light source.
However, it has been known that a wavelength of the light beam (the laser beam) irradiated from the semiconductor laser element is varied in a luminescent wavelength according to a temperature of an environment in which the laser element is placed. Further, each of the semiconductor laser elements is different in a changing amount of the luminescent wavelength with respect to the temperature change. In this case, when the temperature is varied in the periphery of each of the semiconductor laser elements and levels of a change with age are different from each other in each of the laser elements, the wavelengths of the light beams emitted from the respective light sources are varied.
Further, since the characteristic of the semiconductor laser element includes a mode hopping phenomenon that the luminescent wavelength is about 1.5 nm changed with respect to the temperature change about 0.1xc2x0 C., it is hard to uniformly align the luminescent wavelength of all the laser elements at a wide environmental temperature range even when aligning the luminescent wavelength under a certain condition.
As mentioned above, in the case that the luminescent wavelength of the light beam irradiated from each of the semiconductor laser elements is changed due to the temperature change, in detection of a horizontal synchronism and definition of a writing position structured such as to arrange a beam detecting sensor for detecting the horizontal synchronism, for example, at a position equivalent to a mirror surface, to detect the fact that the light beam enters into the sensor by emitting the light beam prior to a timing at which the beam passes through the sensor, and to write the image by setting that the light beam is at the same position at the detecting timing and enters into the image area after a fixed time thereafter, an oscillating angle when the light beam is guided to each of the reflection surfaces of the deflecting apparatus becomes a different angle even when the timing at which the light beam enters into the sensor is the same.
That is, since the position of the beam detecting sensor is fixed, the writing position of the image is substantially maintained to a fixed value when writing the image a fixed time after detecting the fact that the light beam enters the sensor in the case that the luminescent wavelength is changed due to the temperature change, however, at a position opposite to the writing position, at which the exposure of the image is finished, there is a problem that a twice times of change (2xc3x97xcex94xcex8) is generated when setting the change amount of the oscillating angle on each of the reflection surfaces in the deflecting apparatus when the light beam scanned to the same place by the change component of the wavelength changing due to the change of the temperature reaches, to xcex94xcex8.
This generates a phenomenon that a color is shifted, a predetermined color can not reproduced and the like, in a color printer apparatus, and there is a problem of reduction of the resolution and generation of jitter caused by changing an outer diameter and a shape of a dot (an assembly of the light beam on the photosensitive body) constituting the image, in a high speed printer apparatus.
An object of the present invention is to provide an exposure apparatus for scanning a plurality of beams, in which difference between the respective beams is reduced and an image is accurately formed, thereby preventing a color shifting or a reduction of resolution from generating.
According to the present invention, there is provided a multi-beam exposure apparatus comprising a plurality of light sources for irradiating light beams having predetermined wavelengths, pre-deflection optical means for applying a predetermined optical characteristic to the light beam irradiated from each of the light sources, deflection means for deflecting the light beam passing through the pre-deflection optical means to a first direction corresponding to a direction in which a rotatably formed reflection surface is rotated at a predetermined speed, image formation optical means for continuously image forming the light beams deflected in the first direction by the deflection means on an image surface, detecting means for detecting at least one of the light beams passing through the image formation optical means and outputting predetermined signals corresponding to the light beams, and optical elements arranged between the deflection means and the detecting means and changing an emission angle in correspondence to a change of a wavelength of the light irradiated from each of a plurality of light sources.
Further, according to the present invention, there is provided a multi-beam exposure apparatus comprising a plurality of light sources, a first optical element for assembling light beams irradiated from the plurality of light sources to one light beam so as to give a predetermined characteristic, deflection means for deflecting the light beams supplied from the first optical element to a first direction corresponding to a direction in which the reflection surface is rotated, a second optical element extended out along the first direction and image forming the light beams deflected from the deflection means to a predetermined position so as to satisfy a function corresponding to a rotation of the reflection surface in the deflection means, detecting means arranged at a distance optically equivalent to a position at which the light beams passing through the second optical element reaches and in an area except an image area in which the light beam passing through the second optical element functions as an image and detecting at least one of the light beams passing through the second optical element so as to output a predetermined signal, and optical elements arranged between the second optical element and the detecting means, changing an emission angle in correspondence to a change of a wavelength of the light irradiated from the plurality of light sources due to a change of a temperature with respect to the first direction and shifting a position to which the light reaches to a direction having the same amount as a position shifting amount on the predetermined image surface generated by the second optical element due to a difference of the wavelength and having an opposite direction, thereby guiding the light having a wavelength which is changed due to a change of the temperature to the same position on the detecting surface of the detecting means in the case that the reflection surface of the deflection means has the same rotational angle.
Still further, according to the present invention, there is provided a multi-beam exposure apparatus comprising a plurality of light sources for irradiating lights having a predetermined wavelength at a predetermined temperature, pre-deflection optical means for assembling lights irradiated from the light sources to one light beam so as to give a predetermined characteristic, deflection means for deflecting a group of lights emitted from the pre-deflection optical means to a first direction, an image formation lens extended out in the first direction and image forming the lights deflected by the deflection means on a predetermined image surface at a uniform speed, detecting means defined at a distance optically equivalent to the predetermined image surface, arranged at a position in which the lights passing through the lens reaches and in an area except an image area among the predetermined image surface and detecting the lights passing through the lens so as to output a predetermined signal, and optical elements arranged on an optical path between the lens and the detecting means, changing an emission angle in correspondence to a change of a wavelength of the light irradiated from the plurality of light sources due to a change of a temperature with respect to the first direction and shifting a position to which the light reaches to a direction having the same amount as a position shifting amount generated by the lens due to a difference of the wavelength and having an opposite direction, thereby reducing a difference of the image of a main scanning direction position at a position opposite to a writing position in the first direction generated by a difference of the writing timing caused by the light being different from a reference wavelength in the wavelength.